1. Field of the Invention
This invention relates to a method for the production of a stop valve for pipes, with a housing which consists of plastic, has two pipe connectors and in which a blocking element, which can be adjusted by an actuation unit between a flow position and a blocking position, is mounted at least by means of bearing rings and seals, wherein plastic is injected around the inner part, which comprises the blocking element as well as the bearing and sealing rings, in the pre-assembled state with the use of an adhesion means to form the housing. The invention also relates to a stop valve produced by this method.
2. Description of Related Art
A method for the production of a stop valve is known, for example, from DE 44 42 979 A1. In this known method, a plastic covering is firstly injected around the ball as well as part of the operating shaft and the lower part of the journal in a first method step. In a subsequent working step, an elastic insert is arranged on the outside of the plastic covering produced in the first sep, between the flange regions and, in the subsequent working step, the valve housing is injected using plastic onto the covering provided with the elastic insert. This method for the production of a plastic housing for a stop valve is very expensive, since a completely two-part valve housing needs to be produced from plastic. The elastic insert which is used is intended to provide separation between the inner covering and the outer valve housing whereas, outside the region of the elastic insert, the two plastics of the inner covering and of the outer valve housing are materially interlocked. In particular owing to the design of the completely two-part valve housing, consisting of the inner covering and the outer valve housing, this known method can scarcely be regarded as economical.
Another method for the production of a ball valve with a housing consisting of plastic is known from EP-0 575 643-B1. To make the plastic housing in this known method, the pre-assembled inner part in the pre-assembled state is placed in an injection mould which creates the outer shape of the housing and the flow cross section of the pipe connectors, then plastic is injected around it. This way of producing the plastic housing, in which the complete valve is divided into an outer part and an inner part, has proved very successful in practice. By virtue of the division into the pre-assembled inner part and the outer part which is to be injected on, the two housing parts are forced to contract relative to each other as the housing made in the injection mould cools, and radically inward shrinkage of the housing, which would entail jamming of the valve ball in the housing, is prevented.
Especially when a stop valve produced in this way is used with pipes having a large diameter of the pipe connectors, and with high feed pressures in the pipes, especially when used as a gas stop valve, it can be found in practice that leakage occurs at the stop valve, even though the stop valve is shut, because the medium to be blocked infiltrates through between the inner part and the outer part, i.e. between the outside of the blocking element as well as the mounting and sealing rings and the inside of the plastic housing. It is absolutely necessary to avoid such leaks, in particular when the fitments are used to block gas lines.
On the basis of this, it is an object of the invention to improve a method for the production of a stop valve for pipes, with a housing which consists of plastic, so that none of the medium to be blocked can enter between the inner part and the outer part of the valve and which furthermore, is simple and inexpensive to carry out. It is another object of the invention to provide a stop valve produced by this method.
The solution to this object for the method is characterized by the method steps:
a) making a moulding, which corresponds essentially to the outer shape of the insert and to the inner shape of the housing in the region of the insert, from a plastic by means of injection moulding;
b) injecting the plastic, which constitutes the insert, onto and into the moulding made in the method step a) while simultaneously forming a welded bond between the plastic of the moulding and the material of the insert;
c) assembling the stop-valve inner part which consists of the valve ball, the bearing rings, the sealing rings, the bearing collar and the insert together with the moulding, and
d) injecting plastic around the inner part pre-assembled in the method step c) while simultaneously forming a welded bond between the plastic of the moulding, which at least partially constitutes the inner shape of the housing, and the material of the housing.
The method according to the invention is distinguished in that the adhesion means, which is used to connect the inner part and the outer part of the stop valve, simultaneously serves as moulding for the outer shape of the insert and the inner shape of the housing in the region of the insert. The plastic moulding produced in the first method step is partially melted, during the subsequent injection around or on to form the insert and the valve housing, by the plastic injected on in each case, so as to create a materially interlocked welded bond between the moulding and the insert, on the one hand, and the moulding and the valve housing, on the other hand,
This materially interlocked bond between the inner part and the inside of the housing (the outer part) ensures that the leakages described in the prior art are avoided.
According to a practical embodiment of the invention, a thermoplastic is used as the plastic for the moulding. Thermoplastics have, in this regard, the advantage that the heat released when plastic is injected around the inner part converts them into a plasticized and flowable state, so that they fit very well to the outer contours of the inner part and the inner contours of the outer part, and a weld is induced between them.
According to a preferred embodiment of the invention, an adhesion copolymer is used as the plastic for the moulding. Adhesion copolymers are distinguished in that, with appropriate formulation, they bond very well to other plastics, especially polyethylene, and that when a plastic is injected directly onto an adhesion copolymer, the latter partially melts and the welded bond is formed between the two plastics. Since polyethylene is preferably used as the plastic for the valve housing, adhesion copolymers are indeed highly suitable as the adhesion means.
The stop valve for pipes according to the invention, which is produced by the method described above, is characterized in that the inner part and the housing (outer part) are materially interlocked at least partially via a moulding injection-moulded from a plastic. The materially interlocked bond according to the invention between the inner part of the stop valve and the stop-valve outer part constituting the plastic housing makes it possible, for the first time, to counteract internal leakage of the stop valve which involves the medium blocked by the stop valve entering between the outside of the inner part and the inside of the outer part. In addition, the statics of the overall valve are, durably improved by the materially interlocked bond of the inner part and the plastic covering.
Lastly, the invention proposes that, in the case of a stop valve according to the invention with an insert connecting the bearing rings to one another, a plurality of penetrations are formed in the insert, so as to provide additional anchoring points for a geometrically interlocked bond between the housing (outer part) and the inner part. When the injected moulding used as the adhesion means melts, it enters the penetrations of the insert and hence interlocks geometrically in the insert. Besides the materially interlocked bond due to the welding, a geometrically interlocked bond is hence additionally provided.
Other features and an advantage of the invention can be found in the following description of the appended drawing, in which a longitudinal section through a ball valve is represented by way of example.